This invention relates to transmission lines, and more particularly to active terminators for transmission lines.
As electronic devices operate at higher and higher speeds, transmission-line effects become more prominent. Rapid signal switching can cause cables and longer wiring traces on printed-circuit boards (PCBs) to act as transmission lines. Such lines are treated as transmission lines when the time for a signal to travel from one end to the other end of the line is equal to or more than half of the signal transition (rise or fall) time.
Higher-performance integrated circuits can decrease rise and fall times, while PCB traces remain roughly constant. Thus more signal lines have to be treated as transmission lines in more advanced systems.
Reflections are caused by impedance mismatches between the transmission line and its loads. Reflection are signals that travel along the transmission line, back and forth between the driver and a receiver, causing unwanted signal ringing, overshoot, undershoot, and noise. The quality of signals is thus degraded.
Various techniques are used to minimize reflection and ringing. Terminating resistors, resistor-capacitors, and diodes can be added to the transmission line to match line impedances or clamp the undershoots and overshoots of ringing. Active terminators have also been used.
The parent application disclosed an active termination circuit using complementary metal-oxide-semiconductor (CMOS) transistors. While useful, other, more sophisticated, active termination circuits are desirable. In particular, a smaller, less expensive active-termination circuit is desired. A clamp buffer is desired to increase the voltage drive on the gate of the clamping transistors, allowing these clamping transistors to be reduced in size.
What is desired is a zero-standby-power active termination circuit for a transmission line. A lower-power terminator is desired using CMOS transistors.